1. Field of the Invention
The present invention relates to a method for manufacturing a silicon carbide semiconductor device.
2. Description of the Background Art
In manufacturing a semiconductor device, a step of selectively forming an impurity region in a semiconductor substrate is required. For example, when forming an n channel type MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a step of forming a p type region in a portion of an n type semiconductor substrate and then forming an n+ type region in a portion of the p type region is often performed to obtain an npn structure. In other words, double impurity regions different from each other in terms of spreading are formed.
In the case where a silicon substrate is used as the semiconductor substrate, spreading of an impurity region can be adjusted by means of diffusion of impurity. Hence, a double diffusion method utilizing this has been widely used.
Meanwhile, in the case where a silicon carbide substrate is used as the semiconductor substrate, a diffusion coefficient for an impurity is small. This makes it difficult to adjust spreading of an impurity region by means of diffusion of impurity.
In other words, when a region having ions implanted therein is subjected to activation annealing, the region will be formed into an impurity region with almost no change. Hence, the double diffusion method cannot be used.
In view of the above, for example, the following method is disclosed in Japanese Patent Laying-Open No. 2008-147576 (Patent Literature 1). Specifically, first, an ion implantation mask made of tungsten is formed on a silicon carbide substrate. Then, ions of an n type impurity are implanted into the silicon carbide substrate. Thereafter, a portion of the ion implantation mask is etched to expose a larger area of the silicon carbide substrate. Then, ions of a p type impurity are implanted thereinto. According to this method, variation in a positional relation between the double impurity regions different from each other in spreading can be reduced by self-alignment. This results in reduced variation in characteristics of the semiconductor device.
However, in the method disclosed in Patent Literature 1, tungsten, which is large in internal stress, is used for the ion implantation mask. Accordingly, a difference in internal stress between the ion implantation mask made of tungsten and the silicon carbide substrate may cause warpage of the silicon carbide substrate. Particularly, it is considered that the warpage in the silicon carbide substrate tends to be large because recent silicon carbide substrates have large areas.
Thus, in the method disclosed in Patent Literature 1, it is difficult to uniformly control an etching width in etching the portion of the ion implantation mask made of tungsten so as to expose a larger area of the silicon carbide substrate. Accordingly, precision in spreading of impurity regions is decreased, disadvantageously.
In view of the above, the present invention has its object to provide a method for manufacturing a silicon carbide semiconductor device so as to increase precision in spreading of impurity regions.